Check Engine P0420 & Air Filter
 

Yesterday I replaced the air filter in my 98 ES (130k miles). Within an hour of the replacement, my CEL came on w/P0420. Now I know that's a cat indication, but it just seems strange that I had no troubles or CELs with the car, and now this comes on at the same time I replace the air filter. I erased the code and will now watch it, but in the meantime, has anyone ever seen a relationship between that code and air flow?

Walt

There is a hose on the top airbox cover on some later models, maybe yours came loose changing the filter. I can't see that causing an 0420 code, possibly a 044# code

I see that hose you mentioned, but it looks to be okay. I've looked at everything in the area, too. All looks normal. I suppose it was just coincidence that the CEL came on shortly after I replaced that filter. It's still off, but I'm a pessimist and expect to see the light again, and I expect to be replacing a cat soon.

Thanks for the response.

Walt

P0420 almost certainly means you need to replace your cat as it is the distinct code for catalyst system efficency below threshold - aka you need a new cat
airbox/intake system have no effect on the cat

how does the car determine the cat is below efficiency [rhetorical ?]

so it could other things too

My guess is there is probably an O2 sensor in front of and behind the cat - it measures the amount of O2 coming and going out - if the ratio is too low, it determines the cat is bad.

if the 02 is wrong, then what

Im on the same boat you are :(

I have the 420 cat code as well for bank1

Im currently looking into junkyards to see if I can get one for a lower price

sigh.... it's not always the cat! you guys trust me on this, you should fully diagnose this before you dump the money. this is a classic thing where people get a code for ''catalytic converter below efficiency" and you take the bait and assume it's even the catalytic converter that is bad when it could be other things or other things AND the cat.
there are tons of articles in magazines and World Wide Web, not to mention the FSM and Haynes Manual, on how to better diagnose the cat.... and if the cat is not bad, then you can more easily find what is happening......

all this talk of cats has me sneezing

This is good advice. You need to read the code, and then make tests before you start buying parts. Otherwise it could be very expensive before you find the problem. Attachment 142330
Attached is a doument that will explain this code in a little more detail, could possibly save you guys a few coins.

yeah the FSM for my 93 has a similar test run you do in the car to trip the conditions on california model 3vz-fe. thank the lord I have the federal spec [and it follows--just the two, upstream oxygen sensors]. i assume the a!!data you just up-l is almost exact copy of the FSM for this person's 1998 es 300.